Reclaiming of lead in form of high purity lead compound from recovered electrode paste slime of dismissed lead batteries and/or of lead minerals

ABSTRACT

An all-wet process for reclaiming the lead content of impure electrode paste or slime from discarded lead batteries and/or lead minerals, in form of high purity lead compound, comprises a) suspending the impure lead containing material in a lead sulphate dissolving aqueous solution of a salt belonging to the group composed of the acetates of sodium, potassium and ammonium; b) adding to the suspension sulphuric acid in an amount sufficient to convert all lead oxides to lead sulphate soluble in the acetate salt solution and slowly adding to the suspension either hydrogen peroxide or a sulphite or bubbling sulphurous anhydride through it, in a measure adapted to reduce any lead dioxide to lead oxide converted eventually to soluble lead sulphate by the sulphuric acid; c) separating a limpid acetate salt solution containing dissolved lead sulphate from a solid phase residue including all undissolved compounds and impurities; d) adding to the separated solution of lead sulphate either carbonate or hydroxide of the same cation of the acetate salt of the lead sulphate dissolving solution for precipitating highly pure lead carbonate/oxycarbonate or lead oxide or hydroxide, respectively, while forming sulphate of the cation, soluble in the acetate salt solution; and e) separating the precipitated high purity lead compound from the acetate salt solution now containing also sulphate of the same cation of the acetate salt. The acetate salt solution containing also sulphate of the same cation of the acetate salt separated from the precipitated compound of lead is re-cycled to step a) and the content of sulphate of the same cation in the solution is maintained below saturation limit by continuously or periodically cooling at least a portion of the solution separated from the precipitated lead compound to cause selective crystallization of sulphate salt of the same cation of the acetate salt and removing it as a by-product. Optionally, the separated solid phase comprising insoluble compounds of lead and/or undissolved concretions of lead compounds is treated in hot concentrated hydroxide of the same cation of the selected acetate salt and converting these compounds of lead and/or undissolved concretions of lead compounds to soluble plumbites, and the separated lead containing alkaline liquor may be added to the limpid acetate solution for precipitating all reclaimable lead in form of high purity lead oxide or hydroxide.

FIELD OF THE INVENTION

This disclosure relates to techniques for reclaiming high purity leadcompound from impure mixtures, like recovered electrode paste slime ofdismissed batteries or lead minerals.

BACKGROUND OF THE INVENTION

Departing from the long established processes for reclaiming valuablelead content from recovered electrode paste of dismissed lead batteriesor lead minerals based on calcination, prior PCT patent application No.PCT/IT2008/000022, in the name of the present applicant, the content ofwhich is herein incorporated by express reference, disclosed a novelprocess almost completely based on wet treatment of the recoveredelectrode paste or lead minerals capable of producing highly purifiedlead carbonate at high yield with much reduced burden of disposing ofnoxious residues and efficient use of reagents. Nevertheless, in orderto use the reclaimed lead for making new battery paste, the leadcarbonate has to be decomposed to lead oxide by heating the carbonate toabout 400-450° C. This is expensive in terms of energy required.

According to the process disclosed in the above-mentioned prior PCTpatent application, the starting material in the form of electrode pasteslime or finely ground and eventually pre-treated lead mineral, isleached using an acid different from sulphuric, adding hydrogen peroxideor other reducing agent or lead dioxide that may be present in thestarting material and sulphuric acid for converting all lead compoundsto insoluble lead sulphate which is separated together with otherinsoluble substances and is thereafter selectively dissolved in anaqueous solution of a solubilizing compound. To the separated clearsolution of lead sulphate is added a carbonate salt for precipitatingcarbonate/oxycarbonate of lead.

Besides the energy required for eventually converting the carbonate tolead oxide, the acid leaching step of the impure starting materialimplies certain costs and treatment plant complexities.

OBJECTIVES AND SUMMARY OF THE INVENTION

With the aim of reducing the lead reclaiming cost in terms of plantinventory of treatment vessels and related agitators, heaters and/orcoolers, filters, overall treatment plant complexity, pumping and energyrequirements, the applicant has found that an outstandingly effectivesimplification of the all wet lead reclaiming process flow is not onlyviable but even more efficient.

The novel approach, unexpectedly found outstandingly efficient consistsin directly suspending the impure starting material in a lead sulphatedissolving aqueous solution of an acetate salt and adding thereto eitherhydrogen peroxide or a sulphite or alternatively bubbling sulphurousanidride through it, in a measure adapted to reduce any lead dioxideexpected to be present in the impure starting material to lead oxide,and sulphuric acid in a measure adapted to convert all lead oxide tolead sulphate that remains dissolved in the selected dissolving saltsolution.

A clear solution containing the dissolved lead sulphate may then beseparated from a solid phase residue that includes all undissolvedimpurities contained in the impure starting material.

Together with the solid phase of all insoluble substances, separatedfrom the lead sulphate dissolving acetate salt solution, depending onthe origin of the impure starting material to be processed, there may bepresent certain compounds of lead such as oxysulphates or other oxidesthat could not be dissolved in the acetate salt solution. Even the leadof these compounds undissolved by the acetate salt solution caneventually be reclaimed if considered economically or for other reasondesirable to do so. This can be carried out by suspending the separatedsolid phase consisting of impurities and insoluble compounds of lead ina concentrated solution of hydroxide of the same cation of the acetatesalt for decomposing and convert these compounds to soluble plumbites,that dissolve in the hot hydroxide solution which may then be separatedfrom the remaining insoluble impurities. The hydroxide solution nowcontaining the residual lead stripped from the previously separatedsolid phase of impurities may be introduced in the liquid acetatesolution containing the lead sulphate in the vessel in which hydroxideof the same cation of the acetate salt is introduced for precipitatingthe lead contained as lead sulphate in the liquid acetate solution inform of lead oxide or hydroxide.

Precipitation of high purity lead compound from the clear lead sulphatesolution may then by effected either by adding to the solution carbonateof the same cation of the acetate salt used for selectively dissolvedlead sulphate for precipitating insoluble carbonate/oxycarbonate oflead, as contemplated in the process disclosed in said prior PCT patentapplication No. PCT/IT2008/000022, or, more preferably, according to analternative embodiment, instead of a carbonate salt, to the clearsolution of lead sulphate is added hydroxide of the same cation of theacetate salt used for selectively dissolved lead sulphate for causingprecipitation of either oxide or hydroxide of lead, depending from thetemperature of the precipitation bath, thus eliminating even the burdenof eventually having to convert the reclaimed lead carbonate to leadoxide by heating the carbonate in an oven.

The applicant has found that whether a carbonate salt or a hydroxide isused for causing it, precipitation of all lead in the solution as highlypure lead compound is practically complete. Therefore, separation of asolid phase of the highly pure lead compound from the acetate solutionis carried out from the same acetate solution in which the impurestarting material had been suspended.

Though the clear solution of acetate salt becomes progressively enrichedof sulphate of the same cation of the acetate salt used for selectivelydissolving lead sulphate, it may be recycled to the suspension step ofthe impure starting material for as long as the sodium sulphateconcentration remains below saturation. When the sulphate concentrationin the clear acetate salt solution approaches saturation, the solutionmay be cooled to about 10° C. for selectively crystallizing andprecipitating solid phase constituted by sulphate of the cation of theacetate salt used, which is recovered by filtering. The clear acetatesalt solution freed of the sulphate salt may then be recycled to thesuspension bath of the impure starting material.

Preferably, before cooling it for selectively crystallizing andprecipitating solid phase constituted by sulphate of the cation of theacetate salt used, the acetate solution is percolated through a columnfilled with chelating resin for sequestering any residual lead ions inthe solution, before cooling the solution in order to produce lead-freesulphate salt, as a by-product, of broader market acceptance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet of the main steps of the process of thisdisclosure for reclaiming high purity lead oxide from dismissed batteryelectrode slime or lead minerals, alternatively for producing pure leadcarbonate/oxycarbonate or pure lead oxide/hydroxide.

FIG. 2 is a simplified diagram of a plant for reclaiming highly purelead compound from dismissed battery electrode slime or lead minerals,alternatively as pure lead carbonate/oxycarbonate or pure leadoxide/hydroxide, according to the alternative process flows of FIG. 1.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring to the flow sheet of FIG. 1, according to a first embodimentof the process of this invention, particularly suited for treating rawelectrode paste slime recovered from crushed dismissed batteries, butusable, with eventual trivial adaptations, also with finely ground leadminerals, for example galena commonly roasted for converting leadsulphite to lead sulphate, anglesite and lesser common minerals, theimpure starting solid material is suspended in an aqueous solution of asalt capable of dissolving lead sulphate, such as for example an acetatesalt of sodium, ammonium, potassium, urea, mono-, di- ortri-ethanolamine.

To this suspension, sulphuric acid in an amount necessary to convert alloxides present in the starting material to sulphate, and a reducingagent chosen among hydrogen peroxide, a sulphite salt and sulphurousanhydride, is gradually added or bubbled through the suspension bath inan amount necessary to reduce any lead dioxide that may be contained inthe starting material (as would be the case with slime from discardedlead batteries) to lead oxide.

Therefore, in this first Step (1) of the process flow sheet of FIG. 1,in the suspension bath of the starting material take place the chemicalreactions that are formally described herein below in view of the factthat they take place simultaneously, causing the conversion of all leadcompounds to sulphate that dissolves in the aqueous solution containingthe specific dissolving salt mentioned above.

The below described reactions make clearly recognizable the existence ofa synergical action that results in a sensible increment of the speed ofthe chemical conversion process, in view of the fact that dissolution oflead sulphate frees the lead oxide to which (for the case of dismissedbattery electrode paste slime) it was originally intimately tied in theelectrode paste, to form compounds of the type 3PbOPbSO₄ and 4PbOPbSO₄,in consequence of which the oxide quickly reacts, transforming itself tosulphate, which in turn dissolves in the acetate salt solution that, inthe considered embodiment, is sodium acetate, while the lead dioxidephysically embedded in concretions of the above-identified oxysulphates,become more easily reached by the reacting species and therefore is morequickly reduced to lead oxide.

The following reactions relate to an exemplary embodiment of reclaiminghigh purity lead compound from electrode paste slime of dismissedbatteries in which lead sulphate is present as measure component (closeto about 60% by weight) in the impure starting material and dissolved inthe aqueous solution of sodium acetate.

Reaction 1: Dissolution of Lead Sulphate

PbSO4 (insoluble)+4CH3COONa→soluble complex of PbSO4

Reaction 2: Dissolution of Lead Oxide

2CH3COONa+H2SO4→2CH3COOH+Na2SO4

PbO (insoluble)+2CH3COOH→Pb(CH3COOH)2 (soluble)+H2O

Pb(CH3COOH)2+Na2SO4→PbSO4 (insoluble)+2CH3COONa

PbSO4 (insoluble)+4CH3COONa→soluble complex of PbSO4

Reaction 3: Reduction of Lead Dioxide and Dissolution

2CH3COONa+H2SO4→2CH3COOH+Na2SO4

PbO2 (insoluble)+H2O2→PbO (insoluble)+H2O+½O2

PbO (insoluble)+2CH3COOH→Pb(CH3COOH)2 (soluble)+H2O

Pb(CH3COOH)2+Na2SO4→PbSO4 (insoluble)+2CH3COONa

PbSO4 (insoluble)+4 CH3COONa→soluble complex of PbSO4

(Optional) Reaction 4: Solubilization of Insoluble Lead CompoundsPossibly Present in the Impure Starting Material Separated Together withImpurities for Reclaiming of Such a Minor Amount of Lead

4PbO*PbSO4 (insoluble)+12NaOH→5Na2PbO2(soluble)+Na2SO4+6H2O

By way of example, in order to process electrode paste slime recoveredfrom crushed discarded lead batteries, an aqueous solution oftri-hydrated sodium acetate dissolved in water in a concentrationcomprised between 37.5 and 54.5% by weight can be satisfactorily used.Sulphuric acid is added in an amount corresponding to or just exceedingthe stoichiometric requirement for converting all lead oxides to leadsulphate as pre-evaluated for the impure starting material to beprocessed. Preferably, after having added the required amount sulphuricacid, to the suspension bath is added hydrogen peroxide the amount ofwhich may also be pre-calculated in terms of the stoichiometricrequirement for reducing the lead dioxide contained in the startingmaterial.

The amount of electrode paste that can be treated in a certain volume ofsolution, depends on the solubilizing capacity of the lead sulphate inthe solution of the selected acetate salt and of the added amount ofsulphuric acid. The ability of dissolve lead sulphate of the acetatesalt solution depends from its salt concentration. By way of example,one liter of aqueous solution with a concentration of 37.5% by weight ofsodium acetate is able to dissolve 100 g of lead sulphate. By increasingthe concentration of the acetate salt, the amount of lead sulphate thatcan be dissolved increases proportionally. The temperature at which theabove described reactions can be carried out in the suspension bath maybe comprised between about 10° C. up to boiling point. The suspensionmay be stirred with a pails or turbine mixer in order to favor breakingdown of lead compound aggregates.

The combination of the chosen operating conditions (type and fineness ofthe starting solid material, type and concentration of the lead sulphatedissolving salt solution, eventual lead dioxide reducing agent addition,temperature, stirring mode) will influence the time needed forcompleting this first step (1) of the all-wet reclaiming process. Thesulphuric acid used for the sulphation of all the lead oxide in thesolution should preferably have a high concentration in order not toexcessively dilute the lead sulphate dissolving solution.

Once the reaction time that, depending on the combination of thenumerous parameters may generally range between 6 and 15 minutes, alimpid acetate solution containing lead sulphate may be separated fromthe solid phase residues, for example by filtration. All insolubleimpurities and substances are therefore separated from the solution(Step 2 of the flow sheet of FIG. 1).

The subsequent reaction conducted in the limpid acetate solutioncontaining substantially all the lead content of the starting materialin form of lead sulphate by adding to the solution a hydroxide of thesame cation of the selected acetate salt (i.e. either sodium, potassiumor ammonium), according to the preferred alternative contemplated forthe Step 3 of the flow sheet of FIG. 1, at a temperature sufficientlyhigh to ensure precipitation of all the lead in solution in form of PbO(of yellow aspect) instead of in form of lead hydroxide (of whiteaspect) produces a selective precipitation of lead oxide because of itsmuch lower solubility than that of lead sulphate. Generally, thecritical temperature is in the vicinity of 70° C., therefore theprecipitation may be carried out at about 72-73° C. (unless for somereason one should prefer to recover a highly purified lead hydroxide,thermally convertible to lead oxide eventually).

The other alternative contemplated for the Step 3 of the flow chart ofFIG. 1, consists in adding to the liquid acetate solution containingsubstantially all the lead content of the starting material in the formof lead sulphate, instead of a hydroxide, a carbonate of the same cationof the selected acetate salt (i.e. either sodium, potassium orammonium), which produces a selective precipitation of lead carbonate ora mixture of lead carbonate and oxycarbonate, because of their muchlower solubility than that of lead sulphate. In this alternativeembodiment, precipitation may be conducted at any temperature comprisedbetween ambient temperature up to boiling point.

Once the reaction of Step 3 is complete, the precipitated lead oxide orhydroxide or carbonate/oxycarbonate is separated by filtration (Step 4of the flow chart of FIG. 1) from the solution while sulphate of thecation of the hydroxide or carbonate used for precipitating the lead asinsoluble oxide (or hydroxide) or carbonate (and/or oxycarbonate)remains in the solution.

The limpid acetate solution, now containing also the sulphate of thesame cation of the acetate salt, can be integrally recycled to thesuspension bath of selective dissolution of the lead sulphate (Step 1)of the process, for as long as the content of sulphate remains belowsaturation (this limit depends primarily on the type of dissolving saltsolution of the lead sulphate and processing conditions).

Of course, precipitation of excess sulphate salt together with the leadoxide or hydroxide or carbonate/oxycarbonate must be prevented.Therefore, excess sulphate salt must be eventually eliminated from thesolution, well before approaching the saturation limit (Step 8 of theflow sheet of FIG. 1). This may be easily done by exploiting thedifferent solubilities at different temperatures of the sulphate salt(i.e. of sodium, potassium or ammonium sulphate) from that of thecorresponding acetate salt for selectively crystallizing the sulphateand separating it from the acetate solution.

The concentration of the aqueous solubilizing salt solution and thetemperature at which lead sulphate dissolution in it is carried out arenot essential parameters because they simply influence the timenecessary for completing the reactions discussed above and the quantityof lead sulphate that may be dissolved in the solution. In practice, ifthe solubilizing solution, after precipitation of the dissolved lead asoxide or hydroxide or carbonate/oxycarbonate, is recycled back andtherefore one's operates with a recycled solution, more and morerecycles may be necessary to complete dissolution of a given quantity oflead sulphate.

The novel approach of this disclosure has proved itself outstandinglysuitable to process electrode paste slimes where the amounts of thethree main lead compounds, namely lead sulphate, lead oxide and leaddioxide, oscillate in the vicinity of a mean value by a range ofvariability of about 2% by weight and this may in practice impede tocalculate exactly the quantity of sulphuric acid solution for convertingto sulphate all lead oxides present in the impure starting material.

Nevertheless, if in conducting the novel process of this disclosuresulphuric acid happens to be added in excess of the stoichiometricallynecessary amount, after having precipitated the pure lead compound byadding a hydroxide or a carbonate of the same cation of the selectedacetate salt, an excess of sulphate of the cation of the added compoundforms compared to the amount strictly relative to the precipitation oflead sulphate, because of the presence in the solution of free sulphuricacid. Vice versa, if sulphuric acid happens to be added in defect of thestoichiometrically necessary amount, incomplete conversion of oxides tosulphate occurs, thus a residual amount of oxide remains undissolved inthe acetate solution when separating the solid impurities. Should thisaccidentally occur, the separated solid phase may be simply reintroducedin the suspension bath to be eventually converted by introducing anexcess of sulphuric acid.

Continuously or intermittently, whenever the sulphate concentration inthe clear acetate salt solution approaches saturation, the solution ispreferably percolated through a column filled with chelating resin forsequestering any residual lead ions in the solution (Step 5 of the flowsheet of FIG. 1), before cooling the solution to about 10° C. forprecipitating a crystalline solid phase (Step 6 of the flow sheet ofFIG. 1), constituted by sulphate of the cation of the acetate salt used,which is recovered by filtering (Step 7 of the flow sheet of FIG. 1).The clear acetate salt solution freed of the sulphate salt may then berecycled to the suspension bath of the impure starting material whilethe lead-free sulphate salt constitutes a marketable by-product.

Herein below several examples are reported solely for illustratingdifferent possible embodiments of the process of this invention withoutin any manner meaning to exclude other possible embodiments.

EXAMPLE 1

80 g of recovered dried electrode paste having a lead content, expressedas metal equivalent, of 72% was treated under stirring with 1000 mlaqueous solution of tri-hydrated sodium acetate at 37.5% by weight, withthe addition of g 12.2 of concentrated sulphuric acid at 94-96% byweight, at the temperature of 83° C. Successively, hydrogen peroxide at32% by weight was slowly added to the suspension (dropwise for about 10minutes) until no further clarification of the suspension was observed.

The hot suspension was then filtered and the separated solid phase wasconstituted by insoluble lead compounds and lead compound concretions,electrode grid fragments and various additives used for making theelectrode paste such as carbon black, barium sulphate, fibers, etc. andimpurities such as sand, plastic materials, etc. The amount of this darkgrey solid phase was about 4-12% by weight of the solid mass of the dryelectrode paste.

The filtered limpid solution containing lead sulphate was stirred at 83°C. adding thereto sodium hydroxide until reaching a practically completeprecipitation of the lead in the form of lead oxide. The suspension wasthereafter filtered separating the precipitate from the solubilizingsolution of sodium acetate now enriched of sodium sulphate that wasrecycled to the stirred lead sulphate dissolution vessel for as long asthe content of sodium sulphate in the solution remained belowsaturation.

When the content of sodium sulphate in the sodium acetate solutionbecame close to the saturation limit, the solution was percolatedthrough a column filled with chelating resins, for example of thecommercial type denominated Chelex-100 or Dowex A-1, though any otherequivalent resin may be used. The resin filler sequestered almostcompletely the surprisingly small quantity of lead ions residuallypresent in the sulphate solution.

Subsequently the purified sulphate solution (practically lead-free) wasslowly cooled to 10° C., under slow stirring, for precipitating acrystalline solid phase constituted by sodium sulphate that was thenrecovered by filtering the suspension, while the clear solution wasrecycled to the dissolution vessel of the lead sulphate.

The filtered lead oxide accurately rinsed with de-ionized water wasdried at 160° C. for as long as reaching constancy of weight.

The separated dark grey solid phase was suspended in sodium hydroxide at40% by weight, at 50° C. for 15 minutes. The separated limpid liquidphase was introduced into the limpid acetate solution containing alsothe lead sulphate, as part of the required amount of sodium hydroxidefor precipitating all lead in solution as lead oxide or hydroxide(according to a preferred embodiment) in consideration of the fact thatalso the lead present in the solution as sodium plumbite converts itselfto lead oxide (or hydroxide).

At the end of the tests, the following mass balance was recorded.

In 80 g amount of recovered electrode paste used in an experiment, therewere 4 g of insoluble substances of dark grey color containing metalliclead and extraneous substances such as sand, carbon black, bariumsulphate and other substances in minor amounts.

The calculated maximum quantity of recoverable lead oxide was of 62.05 gwhile the quantity of lead oxide effectively recovered was of 62.03 gfor a recovery yield of 99.96%.

Chemical analysis of the recovered solid product confirmed that it wasconstituted exclusively by PbO at 99.99% purity, while the sodiumsulphate that was eventually recovered had a purity of about 99.90%.

The following table summarizes relevant conditions, peculiarities andresults of other four exemplary embodiments of the process of Example 1,described in detail above, but with the indicated alternative conditionsand the results that were obtained, always using as starting materialelectrode paste of the same lot recovered from crushed dismissedbatteries.

Lead sulphate Reaction time Obtained dissolving and Precipitatingpurified lead Yield n. solution temperature compound compound % 2 1000ml 10 minutes NaOH Pb(OH)2 99.94 Sodium 65° C. acetate, @37.5% 3 1000 ml 8 minutes NaOH PbO 99.95 Sodium 90° C. acetate, @40.0% 4 1000 ml 12minutes Na2CO3 PbCO3 99.91 Sodium 45° C. acetate, @42.5%

The lead oxide (whether directly produced by the all-wet process orobtained by heating lead carbonate/oxycarbonate produced by the all-wetprocess) is perfectly suitable for preparing electrode pastes for newbatteries.

The practice of the process of this invention using as starting materiala mineral or a mixture of minerals of lead may be substantially similarto the above described embodiments, an essential pre-step being that ofconverting as much as possible any different salt of lead present in themineral to either lead sulphate or to lead oxide. For example, in caseof galena, the most common lead mineral, the mineral should be heated inair, according to common roasting techniques, until oxidizing the leadsulphite to sulphate. The other common mineral anglesite does not needany prior treatment being itself already constituted by lead sulphate.Of course the mineral(s) should be finely ground for facilitating theirprocessing.

FIG. 2 is a schematic diagram of possible embodiments for an industrialplant for reclaiming variable lead in form of high purity compounds fromrecovered electrode paste of dismissed lead batteries and/or finallyground led minerals, eventually pretreated for converting as much of thelead compounds to lead sulphate.

The scheme of FIG. 2 provides a multi-embodiment illustration of thediscussed processing alternatives (though sodium is indicated as theexemplary selected cation of both the acetate salt and of thealternatively added compounds for precipitating the desired purecompound of lead, according to the various alternatives).

In practice, the plant requires essentially three stirred andtemperature-controlled reactors. To a first reactor RAC (1) in which theimpure material is suspended in an aqueous acetate salt solution, isassociated a first solid-liquid separator F(1) for separating the leadsulphate containing solution from the solid phase constituted byinsoluble impurities of the impure starting material.

To a second reactor for precipitating the desired lead compound of highpurity, that in the multiple alternative scheme of FIG. 2 is anyone ofthe reactors RAC (2), RAC (3) and RAC (4), is associated a secondsolid-liquid separator, that is the related one F(2), F(3) and F(4).

The third and last reactor RAC (5) and associated third and lastsolid-liquid separator F(5) are required for at least periodically (ormore preferably continuously) treating the recycling acetate saltsolution and to recycle it to the first sulphate dissolving reactor RAC(1). The treatment consists of selectively crystallizing by virtue ofthe significantly different solubilities of the acetate salt and of thesulphate of the same cation of the acetate salt, introduced in thesecond reactor for precipitating the desired lead compound, and removingit from the system. This step must be performed (continuously orintermittently) in order to prevent saturating the recycling acetatesalt solution with the sulphate of the same cation, which if let tooccur would cause co-precipitation of this salt together with the leadsulphate (making vain the purification process).

According to the preferred embodiment, in order to ensure that the“by-product” sulphate salt (e.g. sodium sulphate) be substantiallylead-free and thus economically marketable, the plant may include anexchange resin column C(1) filled with an appropriate chelating resin,through which the solution, when directed to the selective sulphatecrystallization reactor RAC (5) (whether continuously or periodically)passes, for sequestering residual lead ions that may be present in thesolution.

Of course, the chelating resin filler will gradually loose its activityand periodically a stripping of sequestered lead ions must be carriedout by circulating through the column (C1) acetic acid for a certainperiod of time. The lead ridden stripping solution of acetic acid usedfor this periodical re-activation of the exchange resin, now containinglead acetate, may be “disposed of” by simply introducing it into thefirst reactor RAC (1), as shown by the relative line.

The multi-embodiment plant diagram of FIG. 2, illustrates also theoptional reactor RAC (2bis) in which, if desirable, in consideration ofthe composition of the impure starting material to be processed,residual amount of lead that may remain associated with the separatedsolid phase of impurities, in form of compounds or concretions thatcould not be dissolved during the treatment of the impure material inthe first reactor RAC (1). The separated solid phase is suspended in hotconcentrated solution of hydroxide of the same cation of the selectedacetate salt for dissolving also these compounds of lead or concretionsthereof. The associated liquid-solid separator F (2bis) permits toseparate all non-lead impurities from a liquor of hydroxide containingsodium plumbite dissolved in it which may be conveniently used as partof hydroxide addition in a second reactor RAC (2) or RAC (3) that may beused for precipitating all the lead in the solution as lead oxide orlead hydroxide, according to a preferred embodiment.

1. A process for reclaiming the lead content of impure electrode pasteor slime from discarded lead batteries and/or lead minerals, in form ofhigh purity lead compound, comprising the steps of a) suspending theimpure lead containing material in a lead sulphate dissolving aqueoussolution of a salt belonging to the group composed of the acetates ofsodium, potassium and ammonium; b) adding to the suspension sulphuricacid in an amount sufficient to convert all lead oxides to lead sulphatesoluble in the acetate salt solution and slowly adding to the suspensioneither hydrogen peroxide or a sulphite or bubbling sulphurous anhydridethrough it, in a measure adapted to reduce any lead dioxide to leadoxide converted eventually to soluble lead sulphate by the sulphuricacid; c) separating a limpid acetate salt solution containing dissolvedlead sulphate from a solid phase residue including all undissolvedcompounds and impurities; d) adding to the separated solution of leadsulphate either carbonate or hydroxide of the same cation of the acetatesalt of the lead sulphate dissolving solution for precipitating highlypure lead carbonate/oxycarbonate or lead oxide or hydroxide,respectively, while forming sulphate of the cation, soluble in theacetate salt solution; e) separating the precipitated high purity leadcompound from the acetate salt solution now containing also sulphate ofthe same cation of the acetate salt.
 2. The process of claim 1, whereinsaid acetate salt solution containing also sulphate of the same cationof the acetate salt separated from the precipitated compound of lead isre-cycled to step a) and the increasing content of sulphate of the samecation in the solution is maintained below saturation limit bycontinuously or periodically cooling at least a portion of the solutionseparated from the precipitated lead compound to cause selectivecrystallization of sulphate salt of the same cation of the acetate saltand removing it as a by-product.
 3. The process of claim 2, whereinprior to cooling for selectively precipitating sulphate salt of the samecation of the acetate salt, the solution is contacted with a chelatingresin adapted to sequester any residual lead ions from the solution forselectively precipitating substantially lead-free sulphate salt of thesame cation of the acetate salt, upon cooling.
 4. The process of claim1, wherein the selective dissolution of the lead sulphate is carried outby suspending the impure material in an aqueous solution of sodiumacetate having a concentration comprised between 10 g and 120 g of saltin 100 g of water at a temperature comprised between 20° C. and boilingpoint, for a stirring time comprised between 5 and 180 minutes.
 5. Theprocess of claim 1, wherein selective dissolution of the lead sulphateis carried out by suspending the impure material in an aqueous solutionof ammonium acetate having a concentration comprised between 20 g and120 g of salt in 100 g of water at a temperature comprised between 20°C. and boiling point, for a stirring time comprised between 5 and 180minutes.
 6. The process of claim 1, wherein selective dissolution of thelead sulphate is carried out by suspending the impure material in anaqueous solution of potassium acetate having a concentration comprisedbetween 20 g and 120 g of salt in 100 g of water at a temperaturecomprised between 20° C. and boiling point, for a stirring timecomprised between 5 and 180 minutes.
 7. The process of claim 1, furthercomprising suspending said separated solid phase residue including allundissolved compounds and impurities in hot concentrated hydroxide ofthe same cation of the selected acetate salt for converting and dissolvecompounds of lead or concretions thereof that could not be dissolved inthe acetate salt solution in form of plumbite salt of the hydroxidecation, separating a lead plumbite containing alkaline liquor from asolid phase of impurities contained in the starting material and addingthe separated lead containing liquor to the separated acetate solutionfor precipitating all reclaimable lead as highly pure lead oxide orhydroxide.
 8. A plant for reclaiming the lead content of impureelectrode paste or slime from discarded lead batteries and/or leadminerals, in form of high purity lead compound, comprising: a) a firstreactor (RAC (1)) having stirring and heating means for suspending theimpure material in a lead sulphate dissolving aqueous solution of a saltbelonging to the group composed of the acetates of sodium, potassium andammonium, means for controlled addition of sulphuric acid and means forcontrolled addition of a reagent belonging to the group composed ofhydrogen peroxide, sodium sulphite and sulphurous anhydride; b) a firstsolid-liquid separator (F1) for separating a solid phase constituted byinsoluble compounds of lead and/or undissolved concretions of leadcompounds and impurities from a limpid acetate salt and lead sulphatesolution; c) a second reactor (RAC (2), RAC (3), RAC (4)) adapted tohold the limpid aqueous solution of acetate salt and lead sulphatesolution, having stirring and heating means and means for adding to thesolution either a carbonate or a hydroxide of the same cation of saidacetate salt, for precipitating insoluble carbonate/oxycarbonate of leador lead oxide or hydroxide, respectively, and forming sulphate of thesame cation of the acetate salt, soluble in the aqueous solution; d) asecond solid-liquid separator (F(2), F(3), F(4)) for separating thesolid phase constituted by said precipitated compound of lead from anaqueous solution of acetate salt now containing also sulphate of thesame cation of the acetate salt; e) means for re-cycling the separatedsolution to said first reactor (RAC (1)); f) a third reactor (RAC (5))having stirring means and means for controlled cooling and/or heating ofa continuously or periodically treated portion of said recyclingsolution for selectively crystallizing sulphate of the same cation ofthe acetate salt contained in the solution; g) a third solid-liquidseparator (F(5)) for separating said crystallized sulphate of the samecation of the acetate salt of said treated portion of recycling solutionthat is then recycled to said first reactor (RAC (1)).
 9. The plant ofclaim 8, further comprising a column filled with chelating resin,through which said continuously or periodically treated portion ofrecycling solution is passed for sequestering residual lead ions fromthe solution, before introducing it in said fourth reactor (RAC (5)) forselectively crystallizing sulphate of the same cation of the acetatesalt contained in the solution.
 10. The plant of claim 9, furthercomprising means for periodically stripping sequestered lead ions fromsaid chelating resin by circulating acetic acid through the column andmeans for introducing the circulated amount of acetic acid to said firstreactor (RAC (1)).
 11. The plant of claim 8, further comprising a fourthreactor (RAC (2bis)) having stirring and heating means for suspendingsaid separated solid phase comprising insoluble compounds of lead and/orundissolved concretions of lead compounds in hot concentrated hydroxideof the same cation of the selected acetate salt and converting saidcompounds of lead and/or undissolved concretions of lead compounds tosoluble plumbites; a fourth solid-liquid separator (F (2bis)) forseparating lead containing alkaline liquor from a solid phase ofimpurities; and means for introducing the lead containing alkalineliquor into said second reactor (RAC (2), RAC (3)) for precipitating allreclaimable lead in form of high purity lead oxide or hydroxide.
 12. Theplant of claim 8, further comprising an oven in which the separated leadcompound, if in form of lead carbonate and/or oxycarbonate or hydroxide,is decomposed to lead oxide and carbon dioxide or water, respectively.